Apparatus for the production of foamed resins

ABSTRACT

Die assemblies and processes for extruding a foamable thermoplastic polyvinylaromatic resin through a rectangular die orifice into a zone of lower pressure such that foaming of the resin occurs as it moves through the zone, said zone being defined by a pair of opposing concave surfaces maintained at a temperature lower than the extrusion temperature and curved so that they substantially conform to the corresponding surfaces of the freely expanding resin without exerting a substantial compressive force thereon. The die assemblies and processes are particularly useful for the production of low density foamed boards having smooth surfaces.

United States Patent McCoy et a1. 14 1 Sept. 26, 1972 [54] APPARATUS FORTHE PRODUCTION 3,406,230 10/1968 Baxter et al. ..-...264/51 OF FOAMEDRESINS 3,222,722 12/1965 Reifenhauser ..18/12 F UX [72] lnvemorsi JohnGerald McCoy, Newport; 3,431,163. 3/1969 G lbert ..18/12 F UX am ReesFoster, Cwmbran both of 3,431,164 3/1969 Gilbert ..18/ 12 F UX England3,466,705 9/1969 Richie ..18/12 F UX [73] Assignee: Monsanto ChemicalsLimited, Lon- FOREIGN PATENTS OR APPLICATIONS 9 England 451,864 10/1948Canada ..425/4 [22] Filed: Aug. 17, 1970 Primary Examiner-Robert L.Spicer, Jr. [21] Appl' 64554 Attorney-Herbert B. Roberts, Arthur E.Hoffman and Related US. Application Data Pamela Hogan [62] gogzierfgglo.726,857, May 6, 1968, 57 ABSTRACT Die assemblies and processes forextruding a foamable [.30] F i A li ti P i i D thermoplasticpolyvinylaromatic resin through a rectangular die orifice into a zone oflower pressure May 1967 Great Bmam "23303/67 such that foaming of theresin occurs as it moves through the zone, said zone being defined by apair of [2?] 98.81. ..42 5/4, 425/192,};53/33; opposing goncave surfacesmaintained at a tempera l 425/ ture lower than the extrusion temperatureand curved 1 0 449 6 6 so that they substantially conform to thecorrespond- 264/46 ing surfaces of the freely expanding resin withoutexerting a substantial compressive force thereon. The die assemblies andprocesses are particularly useful for [56] References Clted theproduction of low density foamed boards having UNITED STATES PATENTSSmooth surfaces- 3,349,434 10/1967 Hureau ..425/464 5 Claims, 2 DrawingFigures APPARATUS FOR THE PRODUCTION OF F OAMED RESINS CROSS-REFERENCESTO RELATED APPLICATIONS This is a division of application Ser. No.726,857, filed May 6, 1968, now US. Pat. No. 3,624,192.

BACKGROUND OF THE INVENTION 1 Field of the Invention This applicationrelates to the extrusion of foamable thermoplastic polyvinylaromaticresins and more particularly relates to improved processes for extrudingsuch resins to form foamed products, e.g., low density boards, which arestrong and have smooth surfaces.

2. Description of the Prior Art Because of their excellentheat-insulating and other properties, foamed thermoplastic resins suchas foamed polystyrene are useful industrial products. These products areoften made by molding processes, but extrusion processes are oftenconsidered to be more convenient. However, extrusion processes presentdifficulties in some instances, e.g., in the production of foamed boardof good quality. (The term board is used to describe fiat sheet materialof substantial thickness, usually at least 0.75 inch and often 1.5-4inches thick, such as the sheet materials frequently used as insulatingmedia.) For example, it is difficult to produce an extruded low densityfoamed polystyrene board having a good surface finish.

SUMMARY OF THE INVENTION An object of the invention is to provide novelprocesses for extruding foamable thermoplastic polyvinylaromaticresins.

Another object is to provide novel processes for preparing extrudedfoamed boards, particularly low density boards, having a good surfacefinish.

These and other objects are attained by extruding a foamablethermoplastic polyvinylaromatic resin through a rectangular die orificeinto a zoneof lower pressure such that foaming of the resin occurs as itmoves through the zone, said zone being defined by a pair of opposingconcave surfaces maintained at a temperature lower than the extrusiontemperature (i.e., the temperature of the resin at the die orifice) andcurved so that they substantially conform to the corresponding surfacesof the freely expanding resin without exerting a substantial compressiveforce thereon.

DESCRIPTION OF THE DRAWING An extrusion die assembly of the invention isexemplified (not to scale) in the drawing, in which FIG. 1 is a frontelevation of the assembly and FIG. 2 is a section along the line 2-2 inFIG. 1.

The die comprises a mild steel block 1 having 82 cylindrical channels 2extending into it from oneside. The channels are arranged to communicatewith a rectangular network of three horizontal and 17 verticalintercommunicating slits 3. Each horizontal slit thus has 16 channelsassociated with it, and each vertical slit has two channels. There arealso 32 cylindrical passages 4 extending completely through the block,each passage being positioned centrally in one of the square meshes 5 ofthe network of slits. The network of slits communicates with a zonedefined by the profiled surfaces 6 and 7 of two mild steel plates 8 and9. The length of each of the plates is greater (usually about l.ll.75times greater) than the overall length of the slit network to ensurethat the foamed resin surface is always in contact with the profiledsurfaces during its initial expansion. The concave surfaces of theplates adjacent to the slit network are spaced apart at a distancecorresponding approximately to the width of the slit network. The plates8 and 9 are cut away as shown at 10 and 11 so as to minimize the area ofcontact with the block 1 and thus minimize heat transfer from the dieoutlet to the plates, and they have adjustable clamp mountings (notshown) by means of which they can be set at various distances from eachother as desired. A series of liquid cooling channels (not shown) isdrilled through each of the plates. The die also has means (not shown)for attaching it to the front end of an extruder so that, when the dieis in use, a foamable resin such as foamable polystyrene is fed into thenetwork of slits and thence through the zone into the atmosphere. Theconcave surfaces of the plates are so shaped that they conform to thecorresponding surfaces of the freely expanding resin without exerting asubstantial compressive force thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following example is givento illustrate the invention and is not intended as a limitation thereof.

EXAMPLE Part A Using the die assembly described above in which theoverall length of the slit network is 1.535 inches, the width of thenetwork is 0.785 inch, the width of the individual slits is 0.025 inch,the plates are clamped so that they are 0.79 inch apart at the lips ofthe die orifice and 1.375 inches apart at the end of the expansion zone,and the radius of curvature of the profiled surfaces of each of theplates is 0.5 inch, extrude foamable polystyrene containing 0.25 percentby weight of silica and 10 percent by weight of butane at a pressure of700 psi and a temperature of C. through the die at a flow rate of 15pounds per hour, while maintaining the temperature of the plates at 15C. by circulating cold water through the cooling channels.

The product is a foamed polystyrene board having a width of 2.75 inches,a thickness of 1.625 inches, an overall average density of 1.25 poundsper cubic foot, good strength in both the transverse and longitudinaldirections, and smooth top and bottom surfaces with an excellent finish.

Part B Repeat part A except for extruding the foamable polystyrenethrough the die directly into the atmosphere instead of using the platesto define a zone. The product is a foamed polystyrene board, the densityof which is similar to that of the board of part A, but the surfacefinish of which is inferior in that it is slightly ribbed.

The die used in the practice of the invention is one having an orificefrom which an extruded freely expanding and foaming resin issues with arectangular or substantially rectangular cross section. According to apreferred embodiment of the invention the die is one which incorporatesfeatures designed to increase its resistance to resin flow, e.g., thedies described in British patents 1,034,120, 1,084,000, and 1,098,408and in copending U. S. application Ser. No. 726,061, filed May 2, 1968in the names of William R. Foster and Stanley J. Skinner and assigned tothe assignee of the present application.

British patent 1,034,120 describes a die having extending into it at'itsinlet end a plurality of separate channels communicating with a numberof slits arranged in the form of a network at the outlet end of the die,each slit having a row of channels associated with it. British patent1,084,000 describes a die having a plurality of obstructionstransversely distributed therein so that, when a foamable resin isextruded through the die, the obstructions together offer a substantialresistance to the flow of resin. British patent 1,098,408 describes aparticularly useful type of die which comprises a plurality of channelsextending into it at its inlet end and communicating with a number ofslits arranged to form the meshes of a network atthe outlet end of thedie, each of at least the majority of the meshes having a substantiallycentral passage leading back from the front face of the die to a pointnearer the inlet end of the die (preferably to the inlet end of the die)and not communicating directly with the slits. US. application Ser. No.726,061 filed May 2, 1968 and assigned to the assignee of the presentapplication describes an improved die of the latter type wherein groovesextending from the outi let end of each of at least the majority of thepassages toward the outlet of its surrounding mesh of slits are providedin the front face of the die to assist in ensuring that a strand offoamed resin extruded from each passage substantially fills the spaceformed by the enveloping foamed resin issuing from the surrounding meshof slits.

In general the zone-defining surfaces have a concave curved crosssection that preferably conforms exactly to the corresponding surfacesof the freely expanding and foaming resin. One manner in which it ispossible to machine the surfaces so that they conform exactly is byusing a working drawing produced from a photograph of the foaming resinextruding freely from the die orifice. Exact conformity is notessential, however, and a cross section corresponding to an arc of acircle usually provides a sufficiently good approximation. Other simplegeometric shapes such as part of a parabola can also be used if desired.

I At the die face the zone-defining surfaces are normally spaced apartby a distance equal to or slightly greater than the width of the dieorifice, and from this point they diverge as the distance from the dieincreases, reaching their greatest distance apart where the foamed resinis completely expanded. Preferably they thereafter extend parallel toeach other for a short distance. Their greatest distance apart dependsof course on the overall width of the die orifice, the degree to whichthe foamable resin expands, and to some extent on the type of die whichis used, but it is generally about 1.2-6 times the width of the dieorifice. When the die incorporates features designed to increase theresistance to resin flow, the greatest distance apart of thezone-defining surfaces is usually 1.2-4 times, particularly l.5-3 times,the overall width of the die orifice. For example, when the die is ofthe type described above and in British patent 1,098,408, particularlygood results are obtained using zone-defining surfaces having a greatestdistance apart corresponding to twice the overall width. of the dieorifice.

In general the curvature of the zone-defining surfaces is dependent onthe thickness of the foamed board which it is desired to extrude,thicker boards normally requiring a larger radius of curvature foroptimum results. For example, when a foamed board having a thickness of1.5 inches is to be extruded through a die of the type described aboveand in' British patent 1,098,408, excellent results are obtained wheneach of the zone-defining surfaces has a section corresponding to asegment of a circle having a radius of 0.5 inch.

From the position of maximum expansion, the resin board can be allowedto move through the air for a time while out of contact with othersurfaces, but is is frequently desirable to support the board between apair of flat parallel surfaces spaced apart at a distance correspondingto the thickness of the board. If desired, provision can be made forreducing or eliminating relative movement between the extruding foamedresin and the second pair of surfaces. For example, each of the secondpair of surfaces can be part of an endless belt carried on rollers. Thebelt can be driven so that the part in contact with the resin movesalong with it, or it can be free to move so that it is driven by thefrictional force between it and the moving resin. Preferably themounting of each endless belt is attached to one of the contouredsurfaces so that the belt and the contoured surface move together whenthe position of the latter is adjusted. Alternatively, instead of beingflat, each of this pair of surfaces can be constituted by a roller ofrelatively large diameter, or there can be a series of rollers traversedin succession by the extruding resin.

The polyvinylaromatic resin which is extruded in accordance with theinvention is a polymer of one or more vinyl or vinylidene aromaticmonomers such as styrene, a chlorostyrene, alp'ha-methylstyrene, o-, m-,or p-rnethylstyrene, other aralkylstyrenes, etc., includinginterpolymers of such monomers with one or more copolymerizableethylenically unsaturated monomers such as acrylonitrile,methacrylonitrile, vinyl chloride, 1

vinyl acetate, methyl and other alkyl acrylates and methacrylates, etc.The invention is particularly applicable to polystyrene resins, such aspolystyrene itself or a toughened polystyrene, i.e., a polystyrenehaving physically or chemically combined therewith a minor proportion,e.g., l-l 5 percent by weight, of a natural or synthetic rubber, e.g.,substantially linear or branched polymers of conjugated dienes, such asbutadiene, isoprene, etc., including copolymers thereof with lesseramounts of comonomers such as styrene, acrylonitrile, methylmethacrylate, etc. v

Being foamable, the resin is in admixture with a blowing agent, which ispreferably a normally gaseous substance but which can be a volatileliquid. In many cases the blowing agent is one that is normally gaseousbut which, while under pressure before extrusion, is present in theliquid state. Exemplary of volatile substances that can be used arelower aliphatic hydrocarbons such as ethane, propane, a butane orbutene, a pentane or pentene, etc., lower alkyl halides such as methylchloride, trichloromethane, l ,2-

dichlorotetrafluoroethane, etc.; and inorganic gases such as carbondioxide and nitrogen. the blowing agent can also be a chemical blowingagent, e.g., a bicarbonate such as sodium bicarbonate, ammoniumbicarbonate, etc., or an organic compound that yields nitrogen onheating such as dinitrosopentamethylenediamine, barium azodicarboxylate,etc. The amount of blowing agent employed is' often in the range of13-30%, especially 7-2O percent, based on the weight of the resin. Forexample, excellent results are achieved by the use of 7-15 percent byweight of butane in conjunction with polystyrene.

The foamable resin preferably also contains a nucleating agent, whichassists in the formation of a large number of small cells. theconventional nucleating agents can be employed, e.g., finely dividedinert solids such as silica or alumina, preferably in conjunction withzinc stearate, or small quantities of a substance that decomposes at theextrusion temperature to give a gas. Exemplary of the latter class ofnucleating agents is sodium bicarbonate, optionally used in conjunctionwith a weak acid such as tartaric or citric acid. A small proportion ofthe nucleating agent, e. g., up to 5 percent by weight of the resin, isusually effective.

Since expansion occurs as the foamable resin leaves the die orifice, thedimensions of the orifice are less than the cross section of the desiredproduct. Expansion takes place along both dimensions of the die, butgenerally greater expansion takes place across the width of the die thanalong its length. In this way a board is produced. By choice of asuitable die size, an extruded product of the desired cross section canbe obtained. For example, a foamed board having a width of 4 feet ormore and a thickness of up to perhaps 2 inches can be produced.Generally, the thickness is at least 0.75 inch, e.g., 1.54 inches. Oftenan increase in the dimensions of the board (with a correspondingreduction in density) can be obtained by heating it, preferably byexposing it'to steam or hot water in a suitable container for a fewmoments. Such treatment is usually more effectively conducted after theboard has been exposed to the atmosphere for a day or two.

The surfaces defining the zone of lower pressure are maintained at atemperature lower than the extrusion temperature. Oil or water coolingcan be employed if desired, and it can be applied, e.g., throughchannels within the surfaces. Sometimes, however, air cooling sufficientalthough the backs of the surfaces can be provided with fins in order toincrease the cooling rate. Normally the surfaces of the zone lead offdirectly from the die orifice. It is desirable to prevent as far aspossible the conduction of heat from the extruder to the cooledsurfaces. This can be achieved, e.g., by the use of a thermallyinsulating material or by arranging for the area of contact between thesurfaces and the extruder to be as small as possible.

Cooling of the surfaces causes a certain amount of drag as the resin isextruded so that the viscosity of the resin in contact with the surfacesis much higher than that of the resin within the extruder, and in factthe resin flow through the zone is normally of a plug character. Thetemperature of the zone-defining surfaces (i.e., the average temperatureof the mass of metal) to be used in a particular instance depends partlyon the nature of the resin andany plasticizing efin general temperaturesof l40 C., preferably l00130 C., are suitable. For example, whenfoamable polystyrene is being extruded, the temperature is frequently 1l0-l 30 C., particularly about C.

The pressure within the extruder is sufficient to prevent anysubstantial foaming of the resin until it leaves the die orifice andenters the zone of lower pressure. For example, pressures greater than250 psi, especially 250-5,000 psi, can be employed. Preferably, the

pressure is 3001 ,000 psi.

The processes and die assemblies'of the invention are of particularvalue in the production of extruded foamed material, e.g., board, havinga low density, e. g., 0.9-1 .5 pounds per cubic foot, and a substantialthickness, i.e., at least 1 inch, and e.g., up to about 2.5 inches.However, a broader range of density can be produced, although there isoften little advantage to be gained by increasing it above 2 or 3 poundsper cubic foot. A density close to 1 pound per cubic foot, for instance1l.5, such as about 1.2 pounds per cubic foot, is usually preferable.

As produced from the die, the extruded resin, e.g., a board hassubstantially flat upper and lower surfaces and slightly curved edges.These edges can be trimmed if required, but they are often sufficientlytrue for many purposes.

It is obvious that many variations can be made in the apparatus andprocesses set forth above without departing from the spirit and scope ofthis invention.

What is claimed is:

1. In a die assembly suitable for the production of a foamedthermoplastic resin board, the improvement comprising a die assemblyhaving a rectangular die orifice communicating with a zone defined by apair of opposing concave surfaces that can be cooled and are curved sothat, when a foamable resin is extruded through the die, theysubstantially conform to the corresponding surfaces of the freelyexpanding resin without exerting a substantial compressive forcethereon, the die has extending into it at its inlet end a plurality ofseparate channels communicating with a number of slits arranged in theform of a network at the outlet end of the die, each slit having a rowof channels associated with it.

2. A die assembly of claim 1, wherein each of at least the majority ofthe meshes of the slit network has a substantially central passageleading back from the front face of the die to the inlet end of the dieand not communicating directly with the slits.

3. A die assembly of claim 1, wherein grooves extending from the outletend of each of at least the majority of the passages toward the outletof its surrounding mesh of slits are provided in the front face of thedie to assist in ensuring that a strand of foamed resinsextruded fromeach passage substantially fills the space formed by the envelopingfoamed resin issuing from the surrounding mesh of slits.

4. A die assembly of claim 1, wherein the zone-defining surfaces have aconcave cross section corresponding to an arc of a circle.

5. A die assembly of claim 1, wherein the zone-defining surfaces reachtheir greatest distance apart where the foamed resin is completelyexpanded and thereafter v extend parallel to each other.

1. In a die assembly suitable for the production of a foamedthermoplastic resin board, the improvement comprising a die assemblyhaving a rectangular die orifice communicating with a zone defined by apair of opposing concave surfaces that can be cooled and are curved sothat, when a foamable resin is extruded through the die, theysubstantially conform to the corresponding surfaces of the freelyexpanding resin without exerting a substantial compressive forcethereon, the die has extending into it at its inlet end a plurality ofseparate channels communicating with a number of slits arranged in theform of a network at the outlet end of the die, each slit having a rowof channels associated with it.
 2. A die assembly of claim 1, whereineach of at least the majority of the meshes of the slit network has asubstantially central passage leading back from the front face of thedie to the inlet end of the die and not communicating directly with theslits.
 3. A die assembly of claim 1, wherein grooves extending from theoutlet end of each of at least the majority of the passages toward theoutlet of its surrounding mesh of slits are provided in the front faceof the die to assist in ensuring that a strand of foamed resin extrudedfrom each passage substantially fills the space formed by the envelopingfoamed resin issuing from the surrounding mesh of slits.
 4. A dieassembly of claim 1, wherein the zone-defining surfaces have a concavecross section corresponding to an arc of a circle.
 5. A die assembly ofclaim 1, wherein the zone-defining surfaces reach their greatestdistance apart where the foamed resin is completely expanded andthereafter extend parallel to each other.